-
/
I
NBSm 73-340 C'^)

VOR AIRCRAFT NAVIGATION SYSTEM MEASUREMENTS

Final Report, Task 1

M.G. Arthur

Donald Halford

C.H. Manney, Jr.

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Prepared for:

Department of Defense

Calibration Coordination Group

Redstone Arsenal, Alabama 35809
i
NBSIR 73-340

VOR AIRCRAFT NAVIGATION SYSTEM MEASUREMENTS

Final Report, Task 1

M.G. Arthur

Donald Halford

C.H. Manney, Jr.

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Prepared for:

Department of Defense

Calibration Coordination Group

Redstone Arsenal, Alabama 35809

U.S. DEPARTVIENT OF COMMERCE, Frederick B. Dent, Secretary

NATKDNAL BUREAU OF STANDARDS. Richard W Roberts. Director

TABLE OF CONTENTS

Page

1. Introduction 1

2. Milestone (a): Consultation 2

3. Milestone (b) : System Requirements and Performance 4

4. Milestone (c) : Report Results of Milestones (a), (b) - 8

5. Milestone (d) : Literature Search 8

6. Milestone (e) : Technique Evaluation 14

7. Milestone (f) : Report Results of Milestones (d) , (e) - 16

8. Milestone (g) : Study and Recommendations 16

9. Milestone (h) : Report Results of Milestone (g) 29

10. Milestone (i) : Joint Meeting 30

11. Acknowledgments 30

12. References 31

13. Bibliography 33

Figures 40

iii
LIST OF FIGURES

Page

Figure 1. Standard Source; Digital Divider Method 40

Figure 2. Standard Source; RLC Method 41

Figure 3. Standard Source; Vector Addition Method 42

Figure 4. Standard Source; Phase Meter Method 43

Figure 5. Direct Measurement; Time Interval Counter

Method 44

Figure 6. Direct Measurement; Pulse Area Method 45

Figure 7. Direct Measurement; Three Voltmeter Method- 46

Figure 8. Direct Measurement; Lissajous Pattern

Method 47

Figure 9. Direct Measurement; Power Factor Method 48

Figure 10. Direct Measurement; Null Detector Method 49

Figure 11. Standard VOR Audiofrequency Signal Source-- 50

Figure 12. Precision VOR Phase Meter 51

iv !
VOR AIRCRAFT NAVIGATION SYSTEM MEASUREMENTS

Final Report, Task 1

Abstract

This report summarizes the work performed under

Task 1 of the VHF Omnidirectional Radio Range (VOR)

phase angle measurement standards project. Included

are (a) a review of VOR system requirements and performance data, (b) results of a literature search of

phase angle measurement techniques, (c) evaluation

of phase angle measurement techniques, and (d)

recommended approaches for a VOR phase angle measurement standard. The recommended approaches are to

develop both a standard source of VOR audiofrequency

signal and a precision VOR audiofrequency phase

meter. These techniques are described in moderate

detail

Key Words: Aircraft navigation standards; phase

angle measurement; phase standard; VHF omnidirectional radio range; VOR standards.

V

1. INTRODUCTION

This report summarizes the work performed under Task 1 of the VHF

omnidirectional radio range (VOR) phase angle measurement standards project, Department of Defense Calibration Coordination Group

(DoD/CCG) Project 73-87, National Bureau of Standards (NBS)

Project 2724461. The project objective is to develop engineering

performance standards necessary to establish a national phase

angle standard for VOR application.

The bearing (heading) information provided by the VOR navigation

system is contained in the phase angle difference between two 30-

Hz audiofrequency signals. The accuracy of the bearing information is assured by calibration of VOR test instruments used to

test and maintain VOR system performance. No national standards

exist for phase angle for VOR application; present calibrations

are accomplished with commercial equipment designed for that purpose. This equipment's accuracy is subject to question, especially

as system requirements become more severe because of increasing

traffic in the air lanes. An accurate national standard of phase

angle for VOR application will decrease the uncertainties inherent

in the present VOR test equipment calibration, and will increase

the safety and efficiency of aircraft operations.

The objectives of Task 1 are to identify the problem areas for VOR

phase angle measurement standards, and recommend approaches for

their solution. The objectives of the second task, Task 2, are

to develop VOR phase angle standards and measurement techniques

for audiofrequency circuitry, and, if practicable, for radiofrequency circuitry.

Task 1 was divided into nine parts, called milestones, to identify

the various steps that needed to be taken to complete the task.

These milestones are as follows:

(a) Consult with VOR users, equipment manufacturers, and system

engineers

(b) Compile detailed system requirements and performance data.

(c) Publish a letter report on the results of (a) and (b) , above.

(d) Conduct a literature search of phase angle measurement techniques.

(e) Evaluate the technical merits of techniques found by (d)

above

(f) Publish a letter report on the results of (d) and (e) , above.

(g) Study and suggest an approach or approaches for a VOR phase

angle measurement standard.
(h) Publish a final report on the results of (g) , above.

(i) Hold a joint meeting with DoD/CCG personnel to discuss results

of Task 1.

When the project was originally being set up, it was felt that NBS

would procure VOR signal generators and other VOR test equipment

in order to evaluate their performance and acquire experimental

data for study. This was not done, however, primarily because no

measurement standards, suitable for this purpose, are available.

Indeed, to develop measurement standards is the primary purpose

of this project.

Therefore, the procedure that was followed was to consult with

persons who are knowledgeable about the design, operation, and

performance characteristics of VOR test equipment, and from these

conversations, plus supportive reports and documents, obtain the

required information. It soon became apparent that the shortcomings of present VOR test equipment are fairly well known within

the avionics community. An alternative procedure would have been

to perform error analyses on the various types of VOR test instruments. This did not seem warranted, however, in view of its high

cost and the limited benefit to the project's primary objective.

The work that was performed under Task 1 is described below. This

information is divided according to the milestones given above.

2. MILESTONE (a): CONSULTATION

2.1 Objective : Consult with VOR users, equipment manufacturers, and -system

engineers

.

2.2 Results : The following agencies, organizations, and individuals were consulted to identify the requirements for VOR phase angle measurements and standards, and to gain a clearer understanding of how

these requirements relate to those of the VOR navigation system:

a. Federal Aviation Administration (FAA)

Systems Research and Development Service

Short Distance Navigation Branch

Washington, D.C.

Mr. A.W. Randall, Chief

2
b. Federal Aviation Administration

Avionics Engineering Section

Oklahoma City, Oklahoma

Mr. W.E. Bell, Unit Chief

Mr. Clee M. Hale, Engineer

Mr. Chester Longman (Flight Inspection Office)

c. Federal Aviation Administration

National Aviation Facilities Experimental Center

Atlantic City, New Jersey

Mr. Jack A. Muller, Engineer

d. Federal Aviation Administration

Navigation Measurements Section

Denver, Colorado

Mr. Harry T. Morgensen, Unit Chief

Mr. Tom Annes, Technician-in-Depth

e . U.S. Army

Metrology and Calibration Center

Redstone Arsenal, Alabama

Mr. J.M. Williams, Jr.

f. Radio Technical Commission for Aeronautics (RTCA)

Washington, D.C.

Mr. Robert Fletcher (FAA) , Chairman, RTCA Subcommittee

SC-121

g. Aeronautical Radio, Incorporated (ARINC)

Washington, D.C.

Mr. William S. Smoot, Secretary

h. Ministry of Transport

Navaids Maintenance Engineering

Ottawa, Canada

Mr. D.R. Vroom, Engineer

Mr. R.J. Gilleland, Engineer

i. Collins Radio Corporation

Cedar Rapids, Iowa

Mr. A.D. Vance, Engineer, Avionics Division

Mr. R.B. Willett, Manager, Metrology Standards Lab.

Mr. Ray Nelson, Metrology Standards Lab.

Mr. A. A. Rogers, Manager, Service Division

Mr. Neil Ennis, Service Division

Mr. Bob Wolter, Service Division

3
j. The Bendix Corporation

Avionics Division

Fort Lauderdale, Florida

Mr. Joseph Sawicki, Project Engineer

k. Tel - Instrument Electronics Corporation

Carlstadt, New Jersey

Mr. D.L. Shacher, Vice President/Chief Engineer

1. The Singer Company

Palo Alto Operation

Palo Alto, California

Mr. Gunther U. Sorger, Director -- Advanced Programs

m. Instrument Flight Research Corporation

Wichita, Kansas

Mr. Harold Selim, Chief Engineer

n. Rohde § Schwarz

Passaic, New Jersey

Mr. Carroll Barlow, Sales Engineer

o. Hewlett Packard Company

Palo Alto, California

Mr. Ray Shannon, Engineer

This information was reported in Letter Report No. 1, dated June 15,

1973, but is included here to make this present report complete.

3. MILESTONE (b) : SYSTEM REQUIREMENTS AND PERFORMANCE

3 . 1 Ob j ect ive : Compile detailed system requirements and performance data.

3 . 2 Results : Conversations with the people cited above and a study of the FAA,

RTCA, and ICAO (International Civil Aviation Organization) publications on this subject have provided the following information. The

conclusions drawn from this information are found in Section 8,

below.

4
Sources o£ information are identified in the references at the

end of this report. Square bracketed [ ] numbers identify

these sources. Persons referenced as "private communication"

are those listed in Section 2.2, above. For the most part, these

private communications were oral, having taken place in person,

by telephone, and by correspondence.

This information was reported in Letter Report No. 1 in brief

form, but it is included here in more detail.

a. Operational VOR receivers in aircraft are required to provide

a bearing information accuracy (airborne component error, E ) of ± 3° [1]. The operational VOR ground component error

spread must not exceed 2°, and the time instability must be

no worse than 0.3° per 15-minute period [1]. These requirements can be satisfactorily met only if the instruments used

to perform receiver calibrations or VOR station measurements

have an accuracy of the order of approximately two or three

tenths of a degree or better. New system requirements for

future VOR operations through the 1980 's will impose smaller

error tolerances (e.g., perhaps ± 1.0°) [2], [3]. It appears

at the present time that the accuracy of a national standard

of VOR phase angle, needed to support present and future system requirements, is approximately one or two hundredths of

a degree or better.

b. The bearing indication of operational and test VOR receivers

is calibrated with calibrated VOR signal generators and re

solver zeroing instruments [4] . Signal generators are

calibrated according to RTCA paper 208-53/DO-52 entitled

"Calibration Procedures for Signal Generators Used in the

Testing of VOR and ILS Receivers." Resolvers are zeroed

according to RTCA paper 126 -54/SC61 -65 entitled "Calibration

Procedures for Test Standard Omnibearing Selectors and Omnibearing Selector Test Sets Used in the Testing of VOR Receivers and Omnibearing Selectors." Modern VOR signal

generators have a phase angle resolution of ± 0.01° whereas

the resolution of older generators is approximately ± 0.05°

[5].

c. The bearing adjustment of operational VOR transmitting stations is accomplished with calibrated VOR signal generators

and other ancillary instruments as outlined in FAA procedures

[6]. The accuracy of this adjustment is limited by the calibration accuracy of the VOR signal generator and the resolution of the VOR phase monitor. The VOR monitor provides an

alarm when a 1° change in phase angle difference occurs [7].

d. VOR signal generators used in tests and calibrations are

amplitude modulated VHF radiofrequency signal sources. The

radiofrequency source may be a separated generator unit from

the audiofrequency modulation unit, or it may be incorporated

5
in one instrument with the audiofrequency source [4] . The

essential VOR bearing information is contained in the audiofrequency signal [1], [8], [9], Calibration of the audiofrequency signal at zero phase angle difference between the

reference and the variable phase voltages is accomplished by

means of a "zero indicator" instrument [4] , [10] , [11] . No

attempt is made to calibrate the bearing information contained in the radiofrequency signal per se [12]. The radiofrequency signal is always demodulated to recover the

audiofrequency VOR signal for calibration and testing [13]

.

e. Two types of zero indicator instruments are generally in use

[10]. One uses a null network to indicate when a zero phase

condition exists. The claimed accuracy for this type of

instrument is ± 0.1°. The other uses a phase detector to

measure phase angle difference. The claimed accuracy for

this type of instrument is also ± 0.1° when the phase angle

difference is within ± 1.0° of zero. However, intercompari

sons of VOR zero measurements made with these two different

types of instruments have revealed as great as 0.7° difference in the zero indication [9] , [10] . Neither type of

indicator is a primary standard based on fundamental quantitie s

.

f. The zero indicator instrument, as normally used, is capable

of indicating only an in-phase condition of audiofrequency

reference and variable phase voltages, although the null

network type can be used in conjunction with a phase shifter

to check selected non-zero phase differences and the phase

detector type can be designed to measure other selected nonzero phase angle differences [4] . Phase angle differences

other than zero are presently established by the generating

mechanism, either electrical or electro -mechanical , of VOR

audiofrequency generators [9], [11]. Modern VOR generators

provide a phase angle resolution as small as 0.01° and a claimed accuracy of approximately ± 0.02° to ± 0.03°. Older

electromechanical generators provide a resolution as small

as 0.05° and a claimed accuracy of approximately ± 0.2° to

± 0.5°. Modern generators, using precision clocks and

digital techniques to produce a precise phase angle difference, are known to disagree with the zero indicator type of

instrument by as much as approximately 0.4° at zero degrees

phase angle difference [14].

g. Because present and future VOR test equipment include both

audiofrequency and radiofrequency instruments, means are

needed to test both types of equipment [15]. In general,

means must be provided to measure the phase angle difference

between the reference and variable phase voltages both at

the output port of VOR audiofrequency generators and at the

output port of VOR radiofrequency generators. In the final

analysis, the radiofrequency measurement is the more useful

6
for VOR receiver testing because the test signal applied to

the receiver is at radio frequency [16] . The audiofrequencymeasurement is more useful for testing zero indicator instruments and ground station performance.

h. As can be seen from b, c, and d, above, present VOR test instruments include both active (signal generators) and passive

(zero indicators) devices. A national standard for VOR

phase angle must support both types of instruments [17]

.

This means that both a standard VOR phase METER and a standard VOR signal SOURCE may be required. A VOR audiofrequency

phase meter should be designed to accept input voltages between approximately 0.1 and 5 volts. A radiofrequency phase

meter should accept input voltages between approximately

0.001 and 1 volt. Audiofrequency signal sources should

generate a 1-volt minimum output signal at 30 Hz and 9960 Hz.

Radiofrequency signal sources should generate a minimum of

50,000 microvolts at VOR frequencies [19].

i. A new generation of test equipment which uses new design concepts is becoming available. However, much of the older

test equipment will remain in service. A calibration capability must support both types of equipment [12]

.

j. Operational VOR receivers vary in design and the VOR signal

is not processed in the same way by all the receivers.

Measurement techniques or measurement standards must apply

to all types of receiving techniques [9]

.

k. Operational VOR signals are normally not pure sinusoids.

Distortion and noise originate from a variety of sources such

as (1) mechanical imperfections in the tone wheel generators,

(2) instabilities in digital signal synthesizers, (3) nonlinear amplification, modulation, and demodulation in generators, zero indicators, and receivers, and (4) electromagnetic

interference [19] . According to an inquiry to ARINC from

Deutsche Lufthansa [20] , harmonic distortion can shift VOR

zero per the following schedule:

Residual frequency modulation (FM) can also occur [16]. The

long-range objective of this project should include a solution to the additional measurement problems introduced by

non-ideal signals. Also, the definition of VOR zero should

take distortion into account.

Harmonic

Distortion

Maximum Phase

Shift Error

0.1%

0.3%

1.0%

2.0%

3.0%

± 0.058°

± 0.166°

± 0.58°

± 1.16°

± 1.72°

7
3 . 3 Summary of Requirements :

a. Operational bearing accuracy:

Receivers: ± 3° accuracy,

Ground Station: 2° error spread;

0.3''/15 minutes stability.

b. Operational test equipment accuracy: ± 0.1° to ± 0.2°,

c. National standards accuracy: ± 0.01° to ± 0.02°.

d. Types of operational test equipment needing support:

Audiofrequency signal generators

Radiofrequency signal generators

Zero indicators

.

e. Requirements b and c, above, apply over the complete range

of 0° to 360° azimuth.

f. Requirement d, above, applies for both older and newer types

of test equipment.

g. Requirement d, above, applies to both older and newer types

of receiving techniques.

h. National standards and measurement techniques must apply to

non-ideal (distorted and noisy) VOR signal sources as well

as to ideal waveforms.

4. MILESTONE (c) : REPORT RESULTS OF MILESTONES (a), (b)

4.1 Objective : Publish a letter report on the results of milestones (a) and (b)

,

above.

4.2 Results : The subject information was contained in Letter Report No. 1,

dated June 15, 1973. It has been somewhat expanded in the present

report

.

5. MILESTONE (d) : LITERATURE SEARCH

5 . 1 Objective : Conduct a literature search of phase angle measurement techniques.

8
5.2 Results:

5.2.1 Extent of Search :

An extensive, but by no means exhaustive, literature search was

conducted for phase angle measurement techniques. The search was

confined to those methods that pertain to VOR phase angle metrology.

In general, two approaches apply to this problem, viz., (a) determination of phase angle difference by comparison with a standard

VOR signal source of known characteristics, and (b) direct measurement of phase angle difference by means of an accurate,

calibrated phase meter.

The more useful papers found by this search are appended at the

end of this report under Bibliography. This bibliography is

restricted to documents that describe methods, techniques, equipment, and analyses for either one or both of the two approaches

cited above. Other papers could be included in this list, but

for the most part, they would be redundant to the papers listed.

In addition to these papers, we have acquired and studied other

related papers, e.g., on VOR system design, but they have been

excluded from the present bibliography on phase angle measurement.

The resources used in this literature search are as follows:

a. We searched the card catalog of the Technical Library of the

U.S. Department of Commerce in Boulder, Colorado. The following subjects were included in the search: Angle, Electronic instruments. Electronic measurements. Measurement,

Navigation, Phase, VOR.

b. We searched, for 1971 and 1972, the KWOC Index for technical

reports. This is a computer printout of key words maintained

by the Technical Library of the U.S. Department of Commerce

in Boulder, Colorado. The following key words were included

in the search: Angle, Measurement, Navigation, Phase, Standard, VOR.

c. We scanned the listings under "Phase, Phase Shift and Time

Delay" of the Electromagnetic Metrology Current Awareness

Service (EMMCAS) . We scanned each issue from August 1969

through May 1973, inclusive. EMMCAS is prepared monthly by

the Electromagnetics Division of the National Bureau of

Standards, Boulder, Colorado, 80302.

d. We searched, for 1971, the Permuterm Subject Index (PSI) of

the Institute for Scientific Information. This is a broad,

powerful method of search. The following generic terms were

used: Angle, Phase, Standard, and VOR. NOTE: Three items

attributed to the generic term "phase" were found under the

complete terms "Phase -Meter " and "Phase -Shi ft er . " This

9
illustrates the complications which arise in the PSI. However, we feel the effort is worthwhile, for this method of

search leads to additional references which are relevant but

which likely would not be found in other practical methods

of search.

e. We used the Science Citation Index (SCI) through 1972 to

discover related and useful papers.

f. We searched the entire index for Electronics magazine for

each of the years 1966 through 1971, inclusive. We also

scanned each individual issue for January 1973 through April

1973. The index for 1972 is not yet available.

g. An extensive bibliography on phase shift measurements appears

as Section 11 in an unpublished report (Beatty et al . , 1967)

of the Electromagnetics Division of the National Bureau of

Standards. Most of that bibliography is directed toward

phase metrology for UHF and microwave signals. Only four of

the 156 items in the bibliography were chosen as relevant and

useful for our present study of VOR phase angle measurement

standards

.

Of the eleven sections in this report. Section 2 was the one

specifically directed toward low frequency measurements of

phase (Turgel , 1967). Of the nineteen references in Section

2, we chose eleven as relevant and useful for our present

study

.

Incidentally, Section 2 is the most complete and useful survey of basic methods of phase angle metrology which we found

in our literature search.

h. We searched each of the documents listed in the Bibliography

for references to additional papers which would be relevant

and useful. Several important items were found this way.

There was considerable duplication, triplication, as

might be expected.

i. We scanned several books on electronic navigation systems,

searching for references to literature on methods of phase

angle metrology. We similarly scanned several articles on

VOR. The effort was unrewarding in that it revealed very

few useful references, and all of those few had already been

found in earlier procedures. The books scanned included:

Radio Navigation Systems for Aviation and Maritime Use , edited by W. Bauss . Pergamon Press: New York, 1963

.

Navigation Systems, A Survey of Modern Electronic Aids , edited by G.E. Beck. Van Nostrand Reinhold: London, 1971

.

10
Avionics Navigation Systems , by M. Kayton and W.R. Fried.

John Wiley ^ Sons: New York 1969.

Electronic Avigation Engineering , by P.C. Sandretto,

International Telephone and Telegraph Corporation:

New York, 1958.

5.2.2 Results of Search ;

The literature search did not yield any methods, per se, for

directly determining the phase angle difference of the two 30-Hz

signals contained in the composite VOR audiofrequency signal (i.e.,

the 30-Hz variable phase voltage plus the 9960-Hz subcarrier, frequency modulated by the 30-Hz reference phase voltage) . Neither

did we find any methods for phase difference of these 30-Hz signals as they exist in the VOR radiofrequency signal (i.e., the

radiofrequency carrier amplitude modulated by the composite VOR

audiofrequency signal). If such methods have been described in

the open literature, we have yet to find them. All of the methods

found pertained to a pair of sinusoids of the same frequency.

This would be a natural consequence of the common definition of

phase angle difference, which pertains only to this condition.

Methods that use mechanical or electro -mechanical devices as

critical measurement elements were arbitrarily excluded from our

considerations, for the present time at least, because our belief

that the maximum accuracy obtainable with such devices is poorer

than with all -electric techniques, and that precision automation

methods are more difficult to apply to mechanical devices.

Although none of the methods found dealt with the composite VOR

signal, it is instructive to review the variety of methods that

were found. There are basically four different metrology methods

using the Standard Source approach and six different methods using

the Direct Measurement approach that are applicable to the VOR

problem. These ten methods are briefly described below. This

information was reported in Letter Report No. 1, but is included

here for completeness.

5.2.2.1 Standard Source Methods :

In each of the standard source methods, two 30-Hz signals are

generated by some suitable means, and their phase angle relationship is established by a specific technique as follows:

a. Digital divider method . See figure 1. The 30-Hz signals are

generated by digitally dividing a high frequency signal (e.g.,

1.08 MHz) by a suitable factor (e.g., 36,000). These 30-Hz signals can be shifted in relative phase by altering the delay in a digital phase shifter. Precise incremental shifts (e.g., 0.01

degree and multiples thereof) can be readily produced. See Dreifus,

1971; Drogin, 1967; Krikorian, 1973; Leuenberger, 1971; and Turgel

,

1967 . 11
b. RLC method . See figure 2. A 30-Hz signal drives a network

o£ resistors, inductors, or capacitors of known values. The phase

shift of the output signal, relative to the driving signal, is

computed from the network equations. Switchable sections are employed to obtain the values of phase shift desired. See Haynie

and Rosenfeld, 1963; Heydeman , 1963; Kritz , 1950; Khomyak, 1971;

Park and Cones, 1960; Shkol'nik, 1971; Smirnov, 1970; Turgel,

1967; and Wintle, 1951.

c. Vector addition method . See figure 3. By some means, for

example by methods a or b above, two 30-Hz signals are generated

with a phase angle difference of 90 degrees. Using calibrated

resistive or inductive voltage dividers, phase inverters, and an

adding network, a vector sum of exact portions of the two in-quadrature signals is generated. The phase angle difference between

one of the original signals and the sum signal is computed from

the voltage ratios employed. See Kritz, 1950; Turgel, 1967; and

Wintle, 1951.

d. Phase meter method . See figure 4. Two stable 30-Hz signals

are generated by some suitable means, and a variable network is

provided to shift their relative phase. The phase difference is

measured with an accurate phase meter. See Scott, 1966.

5.2.2.2 Direct Measurement Methods :

e. Zero -crossing detector/time - interval counter method . See

figure 5. A zero -cro ss ing of one signal starts a time -interval

counter. A zero -cross ing of the other signal stops the counter.

The elapsed time is a measure of the phase angle difference.

See Anon., Electronics, May 1970; Drogin, 1967; Drogin, 1971;

Elliot, 1962; Epstein, 1964; Fritz, 1972; Maxwell, 1966; McKinney,

1967; Rinkel, 1956; and Turgel, 1967.

£. Zero -cross ing detector/pul se -area method . See figure 6. A

zero -cross ing of one s ignal starts a train of pulses or initiates

a step change in a d.c. voltage. A zero -crossing of the other

signal stops the pulse train or restores the d.c. voltage to its

original value. An analog integrator measures the area under the

string of pulses or under the single d.c. pulse. The integrator

output is a measure of the phase angle difference. See Ehret,

Wood, and Thompson, 1969; Prater, 1966; Gruner, 1969; Herbst,

1971; Heydeman, 1963; Paull , 1971; Rinkel, 1956; Turgel, 1967;

and Yu, 1958.

g. Three -voltmeter method . See figure 7. The two 30-Hz signal

sources are connected with one common terminal. Three voltages

are measured: (a) the voltage of one of the signals; (b) the voltage of the other signal; and (c) the voltage difference between

the two signals. The phase angle difference between the two

signals is calculated from these three voltages by use of the

cosine law of vector addition. See Pirkle , 1973 , and Scott, 1966 .

12
h. Lissajous pattern method . See figure 8. The two 30-Hz signals are connected to the vertical and horizontal inputs, respectively, of an oscilloscope or oscillograph. The loops of the

pattern configuration are an indication of the phase angle difference. See Heydemann, 1963; Rinkel, 1956; Stickel, 1971; and

Turgel , 1967.

i. Power factor method . See figure 9. The two 30-Hz signals

are connected to the two inputs of a true power meter. Also, the

individual signal voltages, Vi and V2 , are measured. The "effective phase angle difference," 9, is computed by use of the equation

-1 "''i^'z^

e = cos —^—=^—^1 [<vj> <v^>]^

where the angular brackets indicate an average over the time of

one period. The numerator of the equation is the reading of the

true power meter, and the denominator is the product of the rms

values of Vi and V2 . 6 is the phase angle difference between

fundamental components of the 30-Hz signals if the signals are

distort ion -free , or if they have identical waveforms. See

Khomyak, 1971, and Turgel, 1967.

j. Null detector method . See figure 10. One of the two 30-Hz

signals is connected directly to the phase sensitive null detector,

and the other 30-Hz signal is connected through a calibrated phase

shifter. The setting of the phase shifter to produce a null gives

the desired phase angle difference betv/een the two signals. See

Heydeman, 1963; Kritz, 1950; Rinkel, 1956; and Turgel, 1967.

k . Supplemental Procedures for Attaining and Checking Measurement

Accuracy . Often, measurement procedures can be extended to monitor

or verify the accuracy and internal consistency of the above

methods. One such procedure is to check closure around the circle

from 0° to 360° phase difference. Another is to interchange the

two signals, which should give the same result as a change in the

sign of the phase angle. Still another is to use one signal to

represent both signals at 0° phase angle difference. There are

potentially a large number of symmetry and logic operations of

this sort that can provide supplemental checks on the accuracy of

a standard source or of a phase angle meter. See Dreifus, 1971;

Drogin, 1967; Epstein, 1964; Heydeman, 1963; McKinney, 1967;

Paull, 1971; Pirkle , 1973; and Turgel, 1967.

13
6. MILESTONE (e) : TECHNIQUE EVALUATION

6 . 1 Objective : Evaluate the technical merits o£ techniques found in milestone

(d) , above

.

6.2 Results :

The evaluations presented below are first-pass evaluations only,

and are based upon the published material found in the literature.

They are not based upon analyses or experimental testing conducted

at NBS for the purposes of this project. The principal purpose of

these evaluations is to help determine what application, if any,

these techniques might have to the VOR problem. Since none of

these techniques dealt with the composite VOR signal per se, they

would need to be augmented or modified in some manner to be useful to the problem. Thus an in-depth evaluation of each technique,

based upon a thorough technical analysis, is not considered to be

the best use of project funds at this time. A more thorough

analysis of the techniques recommended for Task 2 is given in

Section 8.2.3.

6.2.1 Standard Source Methods :

a. Digital divider method . This method is readily capable of

0.01° accuracy and resolution, and appears capable in principle

of exceeding this figure. It can provide complete coverage of

0° to 360° with uniform accuracy. The principle of this method

has been applied in several instances (e.g., Turgel , 1967, and

Dreifus, 1971), including application to VOR systems (Drogin,

1967, and Neuvo , 1973). Several commercial generators exploit

this method.

This method has the desirable feature of being easily programmable

and automatable, which makes it useful in automatic testing applications. The digital circuits which it uses can be interchanged

one channel with the other, and in other ways can be readily

checked for proper performance and verification of phase angle

accuracy. The cost of implementing this method is not expected

to be excessive.

b. RLC method . This method has the single strong point of conceptual simplicity, but has several weaknesses as follows: (1)

Although the attainable accuracy is 0.01°, precision components

and careful adjustments are required to realize this accuracy.

(2) The method can be used at only a single frequency with little

tolerance for of f -frequency operation, unless a heterodyne scheme

is used (Turgel, 1967). For example, to obtain a 0.01° accuracy,

the 30 -Hz signals must each have a fractional frequency stability

of 111 ppm or better. (3) Although it is conceptually possible,

it would be very expensive and awkward to cover the range of 0°

to 360° in 0.01° steps by switching RLC network sections.

14
c. Vector addition method . This method is potentially capable

o£ excellent accuracy (e.g., 0.0005°) when precision inductive

or resistive voltage dividers are used. It has the disadvantage

that the basic range is 0° to 90°, and inverter networks are required to cover 90° to 360°. The relationship between phase

angle difference and divider ratio is not linear, which causes

the absolute accuracy to vary with phase angle according to the

slope of an arctangent function. The method is not highly sensitive to frequency variations if the quadrature generator is

designed properly. In terms of practicality, speed of operation,

and automatability , this method does not appear as attractive as

the digital divider method.

d. Phase meter method . This method is an indirect way of obtain ing~"arTcnFwir^TTa^^ difference, and its accuracy depends

directly upon the accuracy of the phase meter. In general, this

method will be slower to use, less efficient, less capable of

self -checking using supplemental procedures, and less automatable

than the digital divider method.

6.2.2 Direct Measurement Methods :

e. Zero -cross ing detector/time -interval counter method . This

method utilizes the basic concept of phase angle difference by

measuring the time interval between equivalent points on the two

signal waveforms. The method has been available for some time,

and is used in several commercial meters. It appears capable of

readily providing 0.01° or better resolution or accuracy over the

complete range of 0° to 360°. It is readily automatable, and convenient to use. Its accuracy is degraded by distortion and noise

unless means are included to reduce these effects. The method

uses standard circuits of high performance, and is not expected

to be costly to implement.

f. Zero -crossing de tector/pul se -area method . This method has

similar characteristics to the time - interval counter method except that the measurement accuracy is somewhat less (e.g., 0.1°)

because of the lower attainable accuracy of electronic integrators.

Improved accuracy can be attained with increased averaging time

together with highest quality analog integration techniques. Time

and cost considerations indicate a preference for the time-interval

counter method, above.

g. Three -voltmeter method . Although the accuracy of this

method can be as small as approximately 0.01° to 0.05° for phase

angle differences between -170° and +170°, the accuracy rapidly

degrades seriously for angles near 180°. The cause of this is

the severe loss of sensitivity near 180°. For best results between ± 170°, the measurements must be performed carefully with

accurate (e.g., ± 0.003%) voltmeters or an inductive voltage

divider. The method is not particularly sensitive to frequency

variation, but is susceptible to distortion and noise. It does

not lend itself to rapid or automatable phase measurement. An

ambiguity of 180° is present.

15
h. Lissajous pattern metho d. This is the classic phase angle

measurement method. It, and variations o£ it (Rinkel, 1956),

have been used for many years, and its limitations are well known.

It relies upon a human observer to be the detector, with the resulting error caused by subjective judgment. With a precision

cathode ray oscilloscope tube and carefully adjusted amplifiers,

the accuracy can be very good except near 90° and 270°, but it is

difficult to verify and could remain open to question. Accuracy

varies with the value of phase angle difference, and there is the

ambiguity of 180° to resolve. For certain types of harmonic distortion, special techniques can minimize the measurement error

that would otherwise occur.

i. Power factor method . The principal shortcoming of this method

lies in the difficulty of accurately measuring the time average

of the product of the two 30-Hz signals. In addition, the method

will require accurate square -law devices to measure the power in

each signal. An analysis of this method shows that the calculated

value of phase angle is relatively little affected by harmonic

distortion on the signals (Turgel, 1967). The method appears to

be rarely used.

j . Null meter method . The accuracy of this method depends upon

the accuracy of the calibrated phase meter and the sensitivity of

the null detector. Distortion and noise will greatly reduce its

accuracy. In general, this method will be slower and less convenient than the direct reading methods.

7. MILESTONE (f) : REPORT RESULTS OF MILESTONES (d) , (e)

7.1 Ob j ect ive : Publish a letter report on the results of milestones (d) and (e)

,

above

.

7 . 2 Results :

The subject information was contained in Letter Report No. 1,

dated June 15, 1973. It has been somewhat expanded in the present

report

.

8. MILESTONE (g) : STUDY AND RECOMMENDATIONS

8 . 1 Objective : Study and suggest an approach or approaches for a VOR phase angle

measurement standard.

16
8.2 Results:

8.2.1 Metrological Considerations :

The problem of VOR phase angle metrology can be divided into four

parts; viz., (a) definition of VOR zero and VOR phase angle; (b)

national reference standards of phase angle; (c) accurate and

precise phase angle measurement techniques; and (d) practical and

satisfactory calibration methods for VOR test equipment and VOR

receivers. In this section we discuss the present state of

affairs, from a metrology standpoint, with regard to these four

items, in order to identify the particulars of the problem to be

solved.

8.2.1.1 Definitions ; ,

The present legal definition of VOR zero in the USA is given [1] as follows: "The reference and variable phase modulations are

in phase when the maximum value of the sum of the radio frequency

carrier and the sideband energy due to the amplitude modulation

signal occurs at the same time as the highest instantaneous frequency of the frequency modulation signal." The ICAO wording [8]

is essentially the same. This definition is precise, unambiguous,

and clearly understood, but it is difficult to utilize because of

the following measurement problems:

(1) The definition deals with the radiofrequency VOR signal which

is difficult to generate or measure with precision at the

present state-of-the-art.

(2) The highest instantaneous frequency of the FM signal occurs

at the maximum voltage value of the 30-Hz reference phase

signal, which is a point of zero slope of the 30-Hz sinusoid,

thus making a precise determination of this point very difficult .

(3) None of the present or known future VOR receivers detect the

highest instantaneous radio frequency as a means of decoding

the bearing information on the VOR radiofrequency signal;

they all detect the average instantaneous frequency [9]

.

Thus a standard VOR signal, based on this definition, would

not bear a direct relationship to the receiver process by

which it is decoded, and equivalency criteria would have to

be established.

(4) Distortion and noise present on the received VOR signal may

seriously displace the VOR zero from the undistorted position,

depending upon the nature of the distortion [20].

(5) On the widely used tone wheel type audiofrequency generator,

measurements show that the instantaneous frequency of the 9960-

Hz frequency modulated subcarrier may exhibit more than one

17
peak in the vicinity of the highest instantaneous frequency

because of manufacturing imperfections [9] . Which peak is

the true highest instantaneous frequency may be difficult

to ascertain.

The definition of VOR phase angle difference is implied in Section 3.4 of reference [1]. It is quoted in part as follows: "The

VOR shall radiate a radiofrequency carrier with which are associated two separate 30 -Hz modulations. One of these modulations

shall be such that its phase is independent of the azimuth of the

point of observation (reference phase) . The other modulation

(variable phase) shall be such that ITS PHASE AT THE POINT OF

OBSERVATION DIFFERS FROM THAT OF THE REFERENCE PHASE BY AN ANGLE

EQUAL TO THE MAGNETIC BEARING OF THE POINT OF OBSERVATION WITH

RESPECT TO THE VOR. ..." (Emphasis added.) This definition, per

se, would not appear to raise any measurement problems for this

project. However, it does not take into consideration the effects

of unspecified (arbitrary) distortions of the modulation waveforms.

8.2.1.2 Reference Standards ; 8.2.1.2.1 National Standards ; No national standards of VOR zero or VOR phase angle difference

exist. The only electromagnetic standards or calibration services which the National Bureau of Standards maintains that are

related to this specific measurement requirement are for Voltage,

Impedance (resistance, inductance, and capacitance), and Attenuation [21] . Measurement techniques are not presently available at

NBS for VOR phase angle per se, either for audiofrequency signals

or radiofrequency signals.

8.2.1.2.2 Audiofrequency Standards :

The virtual standard of VOR zero within the FAA and within most

of the aircraft community at the present time is a small group

of audiofrequency null network type zero indicator instruments

[9] which are maintained by their commercial manufacturer, and

which are used to check the performance, by a comparison process,

of other instruments of like kind [22]. They are quite stable,

and a history of their relative performance characteristics, as

a group, has been maintained over a period of approximately twenty

years. These qualifications, along with the claimed accuracy of

± 0.1°, have led to general acceptance of these instruments as

the best available reference standard.

Many (- 800 to 900) of the null network type of zero indicator

instrument are in use within the FAA and the avionics community,

and serve as the local reference standard for individual test

shops [12] . Their calibrations "trace" back to the small group

cited above. The phase meter type of zero indicator is also in

use throughout the avionics community, but does not appear to

enjoy the favor which the former type has [9].

18
8.2.1.2.3 Radiofrequency Standards:

Although the essential VOR bearing information is contained within

the VOR audiofrequency signal, it is carried to an aircraft via a radiofrequency signal which is then demodulated by the VOR receiver. Both the modulation and demodulation processes may introduce bearing errors in the audiofrequency signal used on the

aircraft [12] . This facet of the VOR metrology problem is

essentially one of accurate measurement and calibration techniques,

and should not be one which requires a reference standard in the

sense discussed above for the VOR audiofrequency signal. However,

the definition of VOR zero is expressed in terms of the radiofrequency signal. To provide a reference standard that is based

on this definition would require (1) the generation of a precision

VOR radiofrequency signal of known characteristics, and (2) a precision technique for comparing this signal with those of VOR

test generators used in the field.

No such radiofrequency standard exists [17]. None of the presently

available VOR test equipment is designed to function on a principle

that ties directly to the stated definition. The approach that

generally is followed is to make provision to minimize the parasitic phase shifts in the modulation circuits of the radiofrequency

generator, and to provide a monitor circuit that demodulates the

radiofrequency signal and delivers a close facsimile of the

original audiofrequency signal. The claimed accuracy of modulator/

demodulator circuits in the most modern test equipment, under best

conditions, is approximately ± 0.01° [10]. However, these circuits are known to be susceptible to drift under varying operating

conditions, and the typical accuracy may be as poor as ± 0.1° [23].

8.2.1.3 Measurement Techniques : Many techniques are available for measuring the phase angle difference between two sinusoidal audiofrequency voltages of the same

frequency, and several of these techniques appear capable of extremely high accuracy and precision. However, none of these

techniques is entirely satisfactory for the particular waveforms

of VOR signals for the following reasons:

a. The basic VOR audiofrequency signal components are not of

the same frequency. The variable phase component is at 30 Hz

whereas the reference phase component exists as frequency

modulation sidebands of the 9960-Hz subcarrier. Thus FM

demodulation is required before same - frequency measurement

techniques can be applied.

b. The "working" VOR signal is a VHF radiofrequency carrier,

amplitude modulated by the basic VOR audiofrequency signal.

Thus amplitude demodulation is required before audiofrequency

techniques can be applied.

19
c. The audiofrequency signals from VOR sources often are nonideal, i.e., they contain distortion and noise of a variety

of types, depending upon the particular device. Examples

are harmonic distortion, phase distortion, additive hum, and

additive noise. All known measurement techniques are adversely

affected by noise and distortion to a greater or lesser degree. Also, the definition of phase angle difference becomes

Suspect when applied to complex waveforms.

8.2.1.3.1 VOR Audiofrequency Phase Meters :

The measurement approach that is used by all current VOR audiofrequency phase meters is to demodulate the 9960-Hz subcarrier to

recover the 30 -Hz reference phase signal, and then by some means

determine the phase angle difference between this 30-Hz signal

and the 30-Hz variable phase signal [19] . This technique suffers

from the following problems

:

a. The predetection filter network that separates the 9960-Hz

FM subcarrier signal from the 30-Hz variable phase signal

will introduce a relative phase shift unless its time delay

is the same at both 30 Hz and 9480 through 10440 Hz [19]

.

b. Because of the large frequency deviation ratio of 16, the

major FM sidebands extend relatively far each side of the

9960-Hz subcarrier frequency. The spectral bandwidth is

approximately twice the frequency deviation, i.e., 960 Hz.

If the pre -de tect ion filter networks have a non-linear

phase shift throughout this bandwidth, phase distortion can

occur which will introduce a phase shift in the 30-Hz

reference signal at the output of the demodulator.

c. The FM demodulator network itself may introduce phase distortion of the 9960-Hz FM signal [13].

d. Because the frequency ratio of the 9960-Hz subcarrier to the

30-Hz reference phase modulation is small (332:1), the postdetector filter network, needed to remove the 9960-Hz

component from the detector output, can introduce a significant phase shift in the recovered 30-Hz reference phase

signal [10].

e . Although the phase shifts of the pre -de tect ion and postdetection networks can be evaluated and a correction applied,

the phase shift caused by the detector network cannot be

evaluated by standard means [13]

.

Either (1) some means must be devised to overcome these problems,

or (2) a new technique must be found to circumvent the need to

demodulate the 9960-Hz subcarrier (see Section 8.2.3.2, below).

I

20
8.2.1.3.2 VOR Radiofrequency Phase Meters;

The measurement approach that is used in all current VOR radio

frequency signal generators to determine VOR phase is to demodulate the r£ signal and apply the recovered composite audiofrequency

signal to a VOR audiofrequency phase meter [19] . This technique

is believed to be adequate [13] because (1) the AM demodulator

circuit can be optimized for very linear demodulation of the

radiofrequency signal, and (2) the phase shifts in the post-detector filter network can be made quite linear with frequency

over the VOR audiofrequency range. However, the degree to which

these two objectives are actually met in practice should be carefully tested if this approach is to be continued.

8.2.1.4 Calibration Methods :

We have not made a study of the calibration procedures used for

VOR test equipment. VOR signal generators are presumably calibrated according to procedures given in RTCA paper 208-53/DO-52

[23]. VOR zero indicators are normally not calibrated by the

user since they are considered to be a primary reference standard,

but presumably their manufacturers have procedures for verifying

their performance at the factory [22].

A study of VOR instrument accuracy by National Aviation Facilities

Experimental Center (NAFEC) personnel [4] shows that VOR signal

generators used in repair stations to calibrate airborne receivers

provide average bearing indications within ± 1.58° at the 95%

(2-sigma) probability level. Omnibearing selectors provide

average bearing indications within ± 1.36° with 95% probability.

Although available calibration procedures were apparently considered to be adequate for the needs of that day, the study

revealed that they were not being followed by all shops. A program is needed to improve the calibration activity by (1) increased

surveillance of testing practices, (2) establishment of a single

standard of zero reference, (3) traceability of test equipment

calibration back to the manufacturer's standard, (4) development

of a low-cost bearing checker to check the calibration accuracy

of shop test equipment, and (5) development of a program of

regular periodic testing of signal generators and portable standards . 8.2.2 Characteristics of VOR Test Instruments :

In this section, we discuss briefly the characteristics of VOR

test instruments (audiofrequency signal generators, radiofrequency

signal generators, and zero indicators) that impact the establishment of a VOR phase angle measurement standard.

VOR test equipment is generally of high quality. A great amount

of development and engineering effort has gone into design and

21
production, resulting in instrioments having very good resolution

and stability. However, absolute accuracy is often suspect because

measurement differences between two instruments, much greater

than those expected from claimed accuracies, routinely occur

without adequate explanation or resolution.

The principal source of signal imperfection in VOR audiofrequency

signal generators is the FM modulation process by which the frequency-modulated subcarrier signal is produced [19] . Phase shifts

in the modulator are difficult to evaluate accurately. Other

potential signal imperfections include (1) harmonic distortion on

all three VOR audio frequencies, (2) residual AM on the 9960-Hz

subcarrier, (3) frequency modulation of the 9960-Hz subcarrier

by the 30-Hz variable phase signal, (4) drift in the phase of

either the 30-Hz variable phase or 30-Hz reference phase signals,

and (5) variations in the frequency modulation index from the

design value of 16.

The principal problem in VOR radiofrequency signal generators is

the phase shifts in the amplitude modulation process [19] . The

site of this phase shift is in the modulator network itself, and

the resulting bearing error cannot be accurately measured by

present techniques. Other problems are (1) drift in the phase

shift of the amplitude modulation circuits, (2) bearing errors

caused by amplitude clipping or phase distortion in the generator's

RF stages, and (3) residual FM on the VHF radiofrequency carrier.

The principal source of error in zero indicator instruments is

the FM demodulator. Phase shifts in this circuit cannot be

accurately determined. Other sources of measurement error are

uncorrected phase shifts in the pre -demodulat ion and post-demodulation filter circuits. The claimed accuracy of ± 0.1° is suspect,

but the stability of zero indication is quite good, e.g., better

than 0.1° over many years. The two types of zero indicators (see

Section 3.2, e, above) in general use have consistently shown a 0.7° difference in VOR zero [9], [10].

8.2.3 Recommended Action :

In view of the findings to date, as presented in the preceding

sections, above, we recommend the following steps be taken:

a. Standard VOR Source

The National Bureau of Standards should develop a precision audiofrequency signal source for VOR application based upon the digital

divider method. A description of this method is given in Section

8.2.3.1, below. This source would be maintained at NBS , and

would serve as the national standard of VOR phase angle.

The purposes of this standard source will be, first, to bring

into uniform performance the present and future measurement instrumentation for VOR phase angle, and second, to provide measurement

22
support for improved test instrumentation and improved operational

equipment (VOR receivers and ground stations) that will be required

for the safe and efficient use of the airways.

The application of this standard source will be to calibrate the

zero indicators and other VOR phase meters that serve as primary

standards in calibration facilities of the DoD, FAA, and other

avionics users and suppliers.

The design of this standard source will be Such as to deliver a stable, well -characterized, nearly-ideal VOR audiofrequency signal

that will merit the support of all parties by virtue of its

superior characteristics and thorough evaluation. It will not be

the function of this source to simulate non-ideal signals having

distortion and noise such as may be found in the real -world VOR

environment. The rationale of this statement is discussed

in d, below.

b. Precision Phase Meter

The National Bureau of Standards should develop a precision audiofrequency phase meter for VOR application based upon the time

interval counter method. A description of this method is given

in Section 8.2.3.2, below. This phase meter would be maintained

at NBS, and would serve as a co -standard of VOR phase angle with

the Standard VOR Source. These two approaches complement each

other, and serve as a check on each other. Because they are entirely different approaches, when taken together they provide the

very desirable metrological virtue of having two completely

independent techniques for establishing a national standard of

VOR phase angle.

The purposes of this phase meter are the same as for the Standard

Source. The applications of this phase meter will be, first, to

serve as an independent check on the accuracy of the Standard

Source, and, second, to calibrate the VOR audiofrequency generators

that may serve as primary or transfer standards in calibration

facilities of the DoD, FAA, and other avionics users or suppliers.

This phase meter will be designed primarily to measure undistorted

VOR signals. Its accuracy with distorted signals has not been investigated at this time.

c. VOR Radiofrequency Standards

Measurement standards should be developed to provide measurement

support for VOR radiofrequency test equipment. To develop rf

standards will require the audiofrequency standards described

above. Therefore, development of rf standards should be planned

to begin toward the end of the audiofrequency standards development.

23
d. Distortion Effects

Techniques should be developed for measuring the effects of distortion and noise on the accuracy of VOR test equipment and

operational systems. To do this will first require the capability

of accurately measuring VOR phase angle for nearly-ideal VOR signals. The standards discussed in a and b, above, are for this

purpose. Once these are available, they will provide a foundation upon which work on distorted and noisy VOR signals can

proceed.

e. Definitions

The question of the usefulness of the present definition of VOR

zero should be settled. In at least one person's opinion [24],

one of the "first things" to be done in this VOR project is to

develop a better definition. Although we at NBS do not yet claim

to be sufficiently expert in VOR navigational matters to recommend

a definition change at this time, we do see serious shortcomings

from a metrology point of view in the present definition. We

also know how difficult it is to change a definition and how farranging the impact can be, especially when it could have international implications. However, we believe we can contribute to

the formulation of a better definition, and we are willing to

work with other cognizant parties in doing so.

f. Measurement Verification System

A measurement verification system should be established to transmit the accuracy of the national VOR standard to other test

instruments with a minimum of degradation. This system would

include (1) accurate calibration techniques, (2) standardized

calibration procedures, and (3) suitable calibration schedules.

8.2.3.1 Standard VOR Source :

The digital divider method appears to be the most promising method

for generating a standard source of VOR audiofrequency voltages.

The basic principles behind this method have been described in

the literature (Drogen, 1967, and Neuvo, 1973), and have been

exploited commercially. The method appears to be capable of the

desired accuracy of 0.01° or better, and is capable of covering

the entire range of 360° with uniform accuracy. It has the

desirable feature of being easily programmable and automatable,

which makes it useful in automatic testing applications. The

digital circuits which it uses are such that they can be readily

checked for proper performance and verification of phase angle

accuracy. The method can generate all of the required VOR audiofrequency phase signals, viz., the 30-Hz variable phase, the 30-Hz

reference phase, the 9960-Hz subcarrier, and the 9960-Hz subcarrier

frequency modulated with a 30-Hz signal.

24
A block diagram of the VOR audiofrequency source is shown in

figure 11. The 30-Hz variable phase signal is produced by a digital sinewave generator. The 30-Hz reference phase signal is

produced as a frequency modulation on the 9960-Hz subcarrier by

another digital waveform generator consisting of a subcarrier

argument calculator and a sinewave amplitude generator. The

desired adjustable phase angle difference between the reference

phase and variable phase signals is obtained by pre-setting a down-counter to the desired phase difference, and starting the

reference phase generator and the down-counter simultaneously.

The variable phase generator is started when the down-counter

has reached its zero state.

The down-counter counts zero crossings from the internal precision clock. A basic clock frequency of 1.08 MHz is selected so

that one cycle of this clock signal corresponds to 0.01° phase

angle for a 30-Hz signal. By delaying the start of the variable

waveform generator by the down -counter , any phase angle difference

from 0° to 359.99° can be obtained in 0.01° steps.

The 30-Hz variable phase generator uses a read-only memory (ROM)

in which sinusoid amplitudes are stored. An up/down counter

selects the positive sine value from the ROM, and also controls

the complementing of these sine values to produce the negative

half cycle. Thus the generated waveform is a synthesized approximation of an ideal sinusoid using discrete amplitude segments.

Each segment is represented by an equation of the form

v(i) = cos 27Tf^(t^ + iT) (1)

where i = segment index (0, 1, 2, •••)

f^ = frequency, 30 Hz

t^ = time when v(i) is maximum positive

T = segment time interval.

Signal distortion caused by this approximation is reduced to an

acceptable level by choice of sampling rate, 1/T, from the ROM

and subsequent filtering.

The frequency modulated waveform generator also uses a ROM plus

other circuits. The waveform, to be generated is of the form

V (t) = cos (27Tf t + b sin 2Trf^t) (2)

c c a where f^ = subcarrier frequency, 9960 Hz

b = modulation index, 16

t = running time variable.

25
Discrete amplitude segments that approximate v^Ct) are of the

form

v(n) = cos(2TTf^nT' + b sin ZTTf nT')

a

^ (3)

where n = segment index (0, 1, 2,

T' = segment time interval.

)

The segment interval, T', is made very small (e.g., a few microseconds) to hold the distortion of v^(t) to an acceptable level.

The outputs from the 30 -Hz variable phase generator and the frequency modulated waveform generator are added to obtain the composite VOR audiofrequency signal having the form

where (J) is the phase angle difference between the variable phase

signal (first term) and the reference phase signal (contained in

the argument of the second term)

.

We have not yet made a complete error analysis of this method.

However, preliminary study indicates that circuit components and

techniques are available to implement this method with the desired

accuracy (- 0.01°).

8.2.3.2 Precision VOR Phase Meter : The zero -cro ss ing detector time - interval counter method appears

to be the best method for measuring the phase angle difference between the VOR reference phase and variable phase voltages. The

method we propose is a novel application of well-established

principles involving measuring the time intervals between zero

crossings of the given voltages. In our method, the 30-Hz reference phase signal is not demodulated from the 9960-Hz subcarrier,

but rather the zero crossings of the 9960-Hz frequency-modulated

voltage are measured and compared with those of the 30-Hz variable

phase signal voltage. Also, the method can be used to measure the

phase difference between the two 30-Hz voltages themselves (i.e.,

apart from the 9960-Hz subcarrier). Internal phase shifts can be

measured by using the method itself in a sel f -checking mode. The

method appears to be capable of the desired accuracy over the entire range of 360 degrees. It is programmable and automatable,

thus lending itself to ATE applications.

A block diagram of the precision VOR phase meter is shown in figure 12. The 9960-Hz frequency-modulated subcarrier, containing

the 30-Hz reference phase signal, is fed to zero -cro ssing detector

No. 1. The 30-Hz variable phase signal is fed to zero -crossing

detector No. 2. The output of zero -crossing detector No. 2 activates the initiate flip-flop which (1) enables the pre-set

count -down counter and (2) starts the time interval counter.

V (t) = cos (2'rrf +(j)) + cos(2iTf t + b sin 27Tf t) , (4)

26
The sequence of operations is as follows: The normally open

momentary operate switch zeros the count -down counter and the

time interval counter, and resets the Initiate flip-flop. The

first positive going zero crossing of the 30-Hz variable phase

signal operates the Initiate flip-flop at time t^; this allows

pulses from zero -cro ss ing detector No. 1, occuring AFTER time i to be counted in the count-down counter. Simultaneously, the

time interval counter begins counting pulses from the clock.

o'

starting at time t When the pre-set number of pulses (zero

crossings), k, from zero -crossing detector No. 1 have been counted,

a pulse from the count -down counter turns the time interval counter OFF. This time count is stored or read out, and is a measure

of the time between time t^ and the kth zero crossing of the subcarrier waveform AFTER time t . The pulse from the count -down

o

^ counter also deactivates the zero -cross ing detectors so that the

measurement cycle is not repeated until the operate switch is

again momentarily closed.

Mathematically, the operation of this method is described as

follows Given the variable phase signal, v (t)

,

a v^(t) = cos OJ^t, (5)

and the frequency modulated Subcarrier signal, v"^(t),

v^(t) = cos[a)^t +b sin(w^t+())^)+(})^] , (6)

where V = amplitude of variable phase signal

V =

^c

-

b

=

angular frequency of v (t) , 2tt x 30 Hz

amplitude of subcarrier signal

angular frequency of subcarrier, Ztt x 9960 Hz

modulation index, 16

phase angle of reference phase signal relative to

variable phase signal

phase angle of unmodulated subcarrier at time zero.

Positive going zero crossings of v (t) occur at times t^ , where

a p

w^tp = ZpiT + 37t/2, (7)

and p is an integer (e.g., 0 , 1 , 2 , 3

, • •

• ) • Positive going zero

crossings of v (t) occur at times t , where

CO t + b sin(co t +6 ) + d)^ = 27Tk„ + 37r/2,

c n ^ a n ^r^ ^c n (8)

27
and k is the integral number of positive going zero crossings of

V ft) AFTER time t = t . In the time of one period of v ft)

,

c^ po ^ ^ * k can range from 1 up through 332. In (8),n is an index that relates a given k to a particular point in time; n therefore also

ranges from 1 through 332.

For three selected positive zero crossings, k^ , k^, and k^ (i.e.,

n = 1, 2, and 3) , we have

0) c t^ + b sin(aj^t^ + (f)^) + = + 3tt/2 , (9)

oa^t2 + b sin(w^t2 + (})^) ^ <t>^ = ^^^2 ^'^/^ » (10)

oo^t^ + b sin(a)^t2 + (j)^) + = + 37t/2 . (11)

That is, in this method, there are k^ positive zero crossings of

the subcarrier signal between time t^ and time t^ ; k^ crossings

between t and t^ ; k^ crossings between t^ and t,.

o 2 3 o 3

Algebraically eliminating (j) and 37t/2 from (9) , (10) , and (11)

gives

c

0)

03.(t2-t^) + b[sin(a3^t2 + c})^) -sin(a)^t^ + (j)^)] = 27T(k2-k^) (12)

.^(t^-t^) + b[sin(a)^t3 + (J)^) -sin(co^t^ + (()^) ] = 2Tr(k2-k^). (13)

Eliminating b from (12) and (13) , and solving for cj)^ gives

. . . ^2^h~^S^ - X (X -X ) ^-r " "^fl^i arctan (14)

[X,(X,-X3) - XjCXg-Xg)]

where = h sin ^^^^ '^^2 '^1-^ ^^^^ X2 = is sin f^^i^^-^i) (16)

X3 = sin^ h

^^a'^^2"^l^ ^^^^ X4 = sin^ h '^a'^^3"'^!^ ^^^^ ^5

^6

h a3^(t2-tp (19)

h

^^c^^3"'^l^ (20)

X^ = (k2-kpTr (21)

Xg = (k3-k^)7T. (22)

All of the quantities o)^ , 03^, t^, t2, and t^ are measurable. The

quantities k^, k2 , and k^ are set in the pre -set count -down count<

A small computer can be programmed to calculate from the data.

28
Having calculated cj)^, the modulation index, b, can be computed

from (14) by the relationship

. 2i,(lc -k )

- a.Jt^-t^)

b = . (23)

sin(a)^t2 + ct'-p) " s in (co^t ^

+ (|)^)

Further, having calculated b, the phase angle, (j)^, o£ the unmodulated subcarrier signal, at time t , can be computed from (8)

by the relationship

(j) = 27rk, + 37T/2 - CO t, - b sin((o t-,+(j) ). (24)

C J. C J. 3. J- IT Thus, this method can provide the complete phase information contained within the composite VOR audiofrequency signal.

We have not yet made a complete error analysis of this method.

However, preliminary study indicates that circuit components and

techniques are available to implement this method with the desired

accuracy (- 0.01°). For example, time interval measurements can

be made with an uncertainty of less than 0.1 microseconds. For a 30 Hz signal, this corresponds to a phase angle uncertainty of

0.00108°.

In order to utilize this accuracy, the zero -crossing detectors

and the pre -set count -down counter must each have comparable

accuracy. The design task is to select or develop circuits and

operating procedures that will satisfy the error budget in which

the sum of all error contributions does not exceed the desired

accuracy

.

This method is not immune to the effects of distortions of the VOR

audiofrequency signals. However, it may be capable of correcting

for these effects, or even measuring the phase angles of distortion

products. Further analysis is required to determine if this is true.

9. MILESTONE (h) : REPORT RESULTS OF MILESTONE (g)

9 .1 Ob j ect ive : Publish a final report on the results of (g) , above.

9.2 Results : A brief final report. Letter Report No. 2, was submitted on

September 10, 1973. The material of that report is greatly expanded in the present report.

29
10. MILESTONE (i) : JOINT MEETING

10.1 Objective : Hold a joint meeting with DoD/CCG personnel to discuss results o£

Task 1.

10.2 Results :

The Subject meeting was held September 18, 1973 at the Aeronautical

Center, Federal Aviation Administration, Oklahoma City, Oklahoma.

The minutes of that meeting are being prepared by Mr. J.W. Williams,

of Redstone Arsenal.

11. ACKNOWLEDGMENTS

Appreciation is expressed to the many people within the DoD, FAA,

and the avionics community who have provided information for this

report. These people are cited in Section 2.2, above. Also,

our thanks go to Miss Janet R. Becker of NBS who typed the manuscript, and to Mr. G.R. Reeve, also of NBS, who read the manuscript

for technical accuracy.

30
12. REFERENCES

[1] Selection Order: U.S. National Aviation Standard for the

VORTAC System, Department of Transportation, Federal Aviation Administration, Order 1010.55, June 1, 1970.

[2] RTCA SC-121 report to be issued.

[3] Private communications: Fletcher, Longman, Muller, Schacher.

[4] Muller, J. A., Analysis of VORTAC System Error Accuracy of

Standards Used by Radio (Avionics) Repair Stations, Report

No. NA-69-22 (RD-69-26), Project No. 330-001-05X, Department

of Transportation, Federal Aviation Administration, National

Aviation Facilities Experimental Center, June, 1969.

[5] Private communications: Hale, Sawicki, Schacher, Selim,

Vroom, Vance.

[6] Private communication: Annes

[7] Very High Frequency Omnirange, Manual FV- 201-1, Department

of Transportation, Federal Aviation Administration, Aeronautical Center, April, 1972, p. 5-1.

[8] Aeronautical Telecommunications, International Standards and

Recommended Practices, Annex 10 to the Convention on International Civil Aviation, ICAO, Vol. I, Part I, July, 1972.

[9] Private communications: Bell, Hale, Longman.

[10] Schacher, D., New Test Equipment for Transponder and VOR/ILS,

Paper 710489, Society of Automotive Engineers, National

Business Aircraft Meeting, Wichita, Kansas, March 24-26, 1971.

[11

[12

Vance, A.G., A Study of VOR Zero, April 30, 1964 (unpublished).

Private communication: Bell, Hale, Longman, Morgensen,

Sawicki, Schacher, Selim, Vance, Vroom, Willett.

[13] Private communications : Sawicki, Selim.

[14] Private communicat ion s : Bell , Hale, Sawicki.

[15] Private communications

Hale , Selim, Williams.

[16] Private communication

Selim.

[17] Private communications

Bell

Williams

[18] Private communications

Hale , Sel im.

[19] Private communications

Bell , Schacher, Selim,

31
[20] ATA Cat. No. 34-55-01, obtained from W. Smoot.

[21] Calibration and Test Services of the National Bureau of

Standards, NBS Special Publication 250, December, 1970

(updated through February, 1973)

.

[22] Private communications: Vance, Willett.

[23] Calibration Procedures for Signal Generators Used in the

Testing of VOR and ILS Receivers, Paper 208 -53/DO-52 , Radio

Technical Commission for Aeronautics, October 1, 1970

(second printing)

.

[24] Private communication: Fletcher.

32
13. BIBLIOGRAPHY

13.1 Standard Source Approach :

Lecture Notes for the Phase Shift Measurements Seminar

Beatty, R.W., D.A. Ellerbruch, D.H. Russell, G.E. Schafer,

R.S. Turgel, J. P. Wakefield, and T.L. Zapf

NBS Report (unpublished)

June 1967, approx. 220 pages

An Accurate Analog Ferrite Phase Shifter

Boyd, C.R. , Jr.

.

1971 lEEE-GMTT International Microwave Symposium Digest of

Technical Papers, Washington, DC, USA

(IEEE: New York, USA)

1971, 104-105

A Precision Pul se -Operated Electronic Phase Shifter and Frequency

Translator

Barnes, J., and A. Wainwright

Proc. IEEE

Dec. 1965, Vol. 53, No. 12, 2143-2144

Digital Phase Control by Harmonic Locking

Cullen, A.L., and A.H. Atani

Electron. Lett. (GB)

Oct. 7, 1971, Vol. 7, No. 20, 609-610

Design and Realization of a Phase -Shifter with Calibration

Independent of Frequency

Castagnos, R. , and P. Duvert

Mesures

(Article: French, Abstract: English § French)

Dec. 1971, Vol. 36, No. 12, 93

Raising the Resolution of Digital Phase Shifters

Dreifus, B.M.

Measurement Techniques

Apr. 1971, Vol. 14, No. 4, 614-618

Digital Phase Shifter Tests Navigation System

Drogin, E.M.

Electronics

Feb. 20, 1967, Vol. 40, No. 4, 104-108

Phase Splitters

Kovalenko, G.G.

Measurement Techniques

Dec. 1971, Vol. 14, No. 12, 1871-1873

Precision Phasemeter for Audio Frequencies

Kritz , J.

Electronics

Oct. 1950, Vol. 23, No. 10, 102-106

33
Phase -Shifter for Phase Locked Loops

Krikorian, J.S., Jr.

EDN

Mar. 20, 1973, Vol. 18, No. 6, 86-87

Digital Phase Shifter Maintains Quadrature

Leuenberger, K.

Electronics

Dec. 20, 1971, Vol. 44, No. 26, 54-55

Harmonic Content of Sine Waves Approximated by Straight -Line

Segments

Mat he son, L.D.

QST

June 1973, Vol. 57, No. 6, 46

Phase Angle Master Standard for 400 Cycles per Second

Park, J.H., and H.N. Cones

NBS J. Research, Sec. C

July-September 1960, Vol. 64C, No. 3, 229-240

A Novel 90° Phase Shifter for Square Waves

Pedar, A., and P.E. Sankaranarayanan

International J. Electronics

June 1971, Vol. 30, No. 6 (In English)

Reference Phase Shifter for Testing Electronic Phase Meters

Type EF-1 (F2-1)

Smirnov, G.P.

Measurement Techniques

Apr. 1970, No. 4, 597-599

Low Frequency Measurements (of Phase Shift)

Turgel, R.S.

NBS Report (unpublished)

June 1967, Chap. 2, 2-1 to 2-30

On the Definition of Concepts of Amplitude, Phase and

Instantaneous Frequency of a Signal

Vakman , D. Ye.

Radio Engineering § Electronic Physics

May 1972, Vol. 17, No. 5, 754-759

Precision Calibrator for L.F. Phase-Meters

Wintle, M.F.

Wireless Engineer

July 1951, Vol. 28, 197-208

The Application of Digital Techniques to a VOR Signal Generator

Neuvo , Y.

IEEE Trans. Aerospace ^ Electronic Systems

Jan. 1973, Vol. AES-9, No. 1, 38-42

34
13.2 Direct Measurement Approach : Phase Detected AutomaticallyElect ronics

September 30, 1968, Vol. 41, No. 20, 176

$1000 Converter Tells All the Angles

Electronics

April 28, 1969, Vol. 42, No. 9, 129-130

Meter Finds Phase Angle Automatically

Electronics

May 25, 1970, Vol. 43, No. 11, 148-150

Telling All the Angles

Electronics

March 2, 1970, Vol. 43, No. 5, 168-172

Digital Computer Controls Accurate Phase and Delay Measurements

Ashworth, H.

Can. Electron. Eng. (Canada)

Dec. 1971, Vol. 15, No. 12, 18-20, 31

Lecture Notes for the Phase Shift Measurements Seminar

Beatty, R.W., D.A. Ellerbruch, D.H. Russell, G.E. Schafer,

R.S. Turgel, J. P. Wakefield, and T.L. Zapf

NBS Report (unpublished)

June 1967, approx. 220 pages

A Phase Meter for Fixed Frequency Measurements

Bortolotti, G., F. Forattini, and S. Gottardi

Antenna (Italy)

June 1971, Vol. 43, No. 6, 227-229 (In Italian)

Error of Measurements Made by Means of a Digital Frequency Meter

with a Phasemeter Attachment

Borisochkin, V.V. , and V.I. Nesturlei

(Pribory Tekh. Eksp. USSR, p. 239, in Russian)

English translation: Instrm. § Exp. Tech. (USA)

May-June 1971, Vol. 14, No. 3, Pt . 2, 942-943.

Digital ICs + VOR = Simpler Navigation

Breikss, I.

Electronics

March 15, 1971, Vol. 44, No. 6, 80-84

Steering a Course to Safer Air Travel

Drogin, E.M.

Electronics

Nov. 27, 1967, Vol. 40, No. 24, 95-102

35
Digital VOR

Drogin, E.M.

Electronics

June 7, 1971, Vol. 44, No. 12, 6

A High-Precision Direct -Reading Loss and Phase Measuring Set for

Carrier Frequencies

Elliot, J.S.

Bell System Technical Journal

Sept. 1962, Vol. 41, 1493-1517

Digitized Phasemeter

Epstein, W.S.

NBS J. Research, Sec. C

Dec. 1964, Vol. 68C, No. 4, 223-226

Linear Integrated Circuit Phase Meter

Ehret, R.L., L.E. Wood, and M.C. Thompson, Jr.

IEEE Trans. Instrumentation ^ Measurement

Sept. 1969, Vol. IM-18, No. 3, 157-160

A Precision Phase Meter

Prater, R.H.

IEEE Trans. Instrumentation ^ Measurement

March-June 1966, Vol. IM-15, No. 1-2, 9-19

A Digital Phase Measuring Device

Fritz, D.

Scientific Report No. 1

University of Innsbruck Electronic Lab., Innsbruck, Austria

July 15 , 1972 , 20 pages

A Wide Range Frequency and Phase Comparison Circuit

Fritze, B.

Nachr ichtentechnische Zeitschrift

Aug. 1971, Vol. 24, No. 8, 437 (Abstract German ^ English)

Digital Phase-Meter for a Phase-Shift Fluorometer

Gorelik, A.I., and A.V. Fal'tsman

Instrum. § Exp. Tech. (USA)

(Prib. Tekh. Eksp. (USSR), Aug. 1972, Vol. 15, No. 2,

Pt. 1, 109-111)

Mar. -Apr. 1972, Vol. 15, No. 2, Pt. 1, 428-430

Phase Comparator Yields Digital Output

Gruner, R.H.

Electronics

Sept. 15, 1969, Vol. 42, No. 19, 118

Detector Measures Phase Over Full 360° Range

Herbst, C.A.

Electronics

July 19, 1971, Vol. 44, No. 15, 73

36
On the Precise Measurement of Phase Angles

Heydeman , P.

Acustica

1963, Vol. 13, 421-426

An Automated 20-200 ,000-cps Transmission Measuring Set for

Laboratory Use

Haynie, G.D., and P.E. Rosenfeld

Bell System Technical Journal

Nov. 1963, Vol. 42, No. 6, 2501-2531

New Version of the State Standard for Phase Meters

Khomyak, V.A.

Measurement Techniques

May 1971, Vol. 14, No. 5, 744-745

General Classification in Phase Measurements

Kucherenko, G.N.

Measurement Techniques

May 1969, Vol. 12, No. 5, 657-662

Measuring the Phase Shift Between Sinusoidal Oscillations of

Different Frequencies

Luan , N.G., and V.M. Solov'yev

Telecommunications § Radio Engineering

Part II, Radio Engineering- -translat ion of Radiotekhnika

Sept. 1971, Vol. 26, No. 9, 91

Use of Phase Detectors to Measure Phase Shifts Between

Sinusoidal Oscillations of Unequal Frequencies

Luan, N.G., and V.M. Solov'ev

Telecommun. § Radio Eng. Pt. 2 (USA)

(Radiotehnika [USSR] Jan. 1972, Vol. 27, No. 1, 80-81)

Jan. 1972, Vol. 27, No. 1, 116-118

A 5 to 50 MHz Direct -Reading Phase Meter with Hundredth-Degree

Precision

Maxwell, D.E.

IEEE Trans. Instrumentation Measurement

Dec. 1966, Vol. IM-15, No. 4, 304-310

Digitized Low- Frequency Phasemeter Assembled from Logic Modules

McKinney, J.E.

NBS J. Research, Sec. C

July-Sept. 1967, Vol. 71C, No. 3, 227-238

Low Frequency Digital Phasemeter Assembled from Logic Modules

McKinney, J.E.

NBS Technical News Bulletin

Dec. 1967, Vol. 51, No. 12, 263, 272

37
Remote Measurement o£ Phase Angle

Nield, P.N.

Proceedings lEE

May 1972, Vol. 119, No. 5, 548-556

Phase Measuring with Increased Accuracy

Paull, C.J.

Electronic Engineering

July 1971, Vol. 43, No. 521, 52 (Abstracts in English, French

and German)

Two Amplitude Measurements Determine Unknown Phase Angle

Pirkle, S.J.

Electronic Design

April 1, 1973, No. 7, 86

Phase -Angle Voltmeter is Digital

Reizenman, M.J.

Electronics

March 29, 1973, Vol. 46, No. 7, 99

Measurement of Phase

Rinkel , S.

Handbook o£ Electronic Measurements

M. Wind, editor

Polytechnic Inst, of Brooklyn, New York

1956, Vol. II, Chapt. VII, 7-1 to 7-48

A Standard for Accurate Phase -Angle Measurements at Audio

Frequencies

Scott , W.W. , Jr.

NBS Technical Note 347

Oct. 14, 1966, 30 pages

Method of Measuring Phase Shift

Shkol'nik, V.M.

Measurement Techniques

Nov. 1971, Vol. 14, No. 11, 1713-1717

A System for the Precise Calibration of Air Navigational Receivers

Sorger, G.U.

IEEE Trans. Instrumentation § Measurement

Nov. 1972, Vol. IM-21, No. 4, 519-527

A New Principle of CRO Phase Measurement with High Accuracy

Stickel , R.

Elektrotech. Z. (ETZ) A (Germany)

Oct. 1971, Vol. 92, No. 10, 586-591 (In German)

Accurate Phase Measurements on Low-Level Microwave Signals

Sunduchkov, K.S.

Radiotekhnika (USSR) In Russian

English trans.: Telecommun. Radio Eng. Pt. 2 (USA)

Nov. 1970, Vol. 25, No. 11, 127-130

38
Low Frequency Measurements (of Phase Shift)

Turgel, R.S.

NBS Report (unpublished)

June 1967, Chap. 2, 2-1 to 2-30

Coincident Slicer Measures Phase Directly

Yu, Y.P.

Electronics

September 12, 1958, Vol. 31, No. 37, 99-101

39
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FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1 . PUBLIC ATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA NBSIR 73-340

SHEET

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

VOR Aircraft Navigation System Measurements

r xnax Kepor^/ lasjv x

5. Publication Date November 1973

6. Performing Organization Code

7. AUTHOR(S)

M. G. Arthur, Donald Halford, and C. H. Manney

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs.

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No. 2724461, Task 1

11. Contract/Grant No. Crrz V.>V^V3 Pvri-i XrXOJ • 7"? / J — P7O /

12. Sponsoring Organization Name and Address

DoD/CCG

Redstone Arsenal, Alabama 35809

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15, SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report summarizes the work performed under Task 1 of the VHF

Omnidirectional Radio Range (VOR) phase angle measurement standards

project. Included are (a) a review of VOR system requirements and

performance data, (b) results of a literature search of phase angle

measurement techniques, and (d) recommended approaches for a VOR phase

angle measurement standard. The recommended approaches are to develop

both a standard source of VOR audiofrequency signal and a precision VOR

audiofrequency phase meter. These techniques are described in moderate

detail

.

7

17. KEY WORDS (Alphabetical order, separated by semicolons)

Aircraft navigation standards; phase angle measurement; phase standard;

VHF omnidirectional radio range; VOR standards

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